All of the briskets I have done, have done fairly well. My main focus to keep it moist, is to limit any puncturing, wait until the last minute to slice, and mist it with a juice throughout the cook to help seal the juices/moisture in.
I would also wonder if when you wrapped it with the beef broth if the broth was high in salt? If so, I would assume that the salt may encourage more moisture to be drawn out from the meat, especially the flat since it trend s to be thinner and expose more of it's meat.
I am no expert on brisket by any means, but this has worked well for me thus far.